The proboscis of the female is a very complicated organ, consisting of six separate bristle-like pieces or setæ, all nearly of the same length. Two of these are somewhat curved near the end, and provided on the inside with fine teeth; another pair consists of very thin lancet-pointed instruments; then comes another lancet-pointed seta, very sharp at the end, and traversed by a canal; next, a stouter and darker-looking tube, slit up underneath, which serves to contain the two toothed setæ; and lastly, a stout and broader sheath, also slit up throughout its length beneath, in which all are packed. This sheath has two lobes at the end, and has some resemblance to the proboscis of the fly, of which organ it is the representative. The gnat lays its eggs in water. The eggs are longish oval, with a kind of neck at the upper end; they are glued together side by side in large numbers, and form a boat-like mass, floating on the surface of the water. The larvæ (Pl. X. fig. 9) are very commonly found skipping through the water, or hanging, as it were, by the tail from the surface. The head is very broad; and arising from each joint of the body are tufts of hairs. Near the end of the body is a tube which communicates with the tracheæ; and when the animal is quiet, this tube is brought to the surface; so that when the animal appears hanging to the surface, it is breathing. In addition to this method of respiration, there are other respiratory organs attached to the last joint of the body, consisting of leaf-like plates; hence these larvæ have an aquatic as well as an aërial respiration: the respiratory or branchial plates also serve as a tail to aid in swimming. Running down the back of the larva will be seen a thin delicate vessel, dilated opposite the thorax, and beating or contracting at regular intervals. This is the dorsal vessel, the dilatation representing the heart of the higher animals; and it serves to propel the colourless blood throughout the body. After several moultings or castings of the skin, to allow of the growth of the insect (for insects grow only in the larval state), the larva becomes transformed into the pupa (Pl. X. fig. 8). In this state the animals still move about in the water, and are often found suspended from its surface by two respiratory tubes, which, however, are not connected with the tail, but arise from the thorax; and the various parts of the perfect insect may be seen through the case or skin, within which they are closely packed. When the pupa has attained its full development, the perfect insect emerges from it, leaving the water to fly about and seek food.